The present invention relates to a cathode, which can be used in molten carbonate fuel cells (hereinafter, frequently referred to as an "MCFC"), and a process for preparing the same. More specifically, the present invention relates to a process for preparing a cathode having a longer life than common cathodes for MCFC by adding alkaline earth metal oxides, for example MgO, SrO, etc., to NiO, which is a main component of cathodes, to reduce the solubility of NiO in the electrolytes of MCFC while maintaining its performance as the common cathode.
Nickel oxide (NiO), which is currently used as a material of common cathodes for MCFC, has been widely used because it is inexpensive and has relatively good electrochemical performance. However, the solubility of NiO in Li.sub.2 CO.sub.3 /K.sub.2 CO.sub.3 molten carbonate, an electrolyte of MCFC, is relatively high. Thus, using NiO presents the problem that, as the operating time of a cell increases, NiO is dissolved in the electrolyte thereof and then is precipitated as Ni metal which can cause a short circuit within the cell. The life of the cell is thereby shortened. Accordingly, to achieve continuous operation over 40,000 hours which is a development goal in an MCFC commercial system, the dissolution problem of cathodes must be solved. With this goal in mind, research has been steadily conducted to solve the dissolution problem of cathodes in the electrolytes of the cells.
First, the method of suppressing cathode dissolution by adjusting the operating conditions of cells has been suggested. The solubility of NiO in electrolytes is dependent on the partial pressure of CO.sub.2 in the gas of cathodes, the operating temperature of cells, the concentration of H.sub.2 O and so on. Therefore, when the operating conditions of cells are adjusted so that the dissolution of the cathodes thereof is reduced, the performance of the cells is also slightly reduced. However, the life of the cells may be extended due to the reduction of cathode dissolution. In other words, cathode dissolution may be suppressed by lowering the partial pressure of CO.sub.2 in gas of the cathodes and increasing the thickness of matrixes to increase the time before a short circuit of the cell occurs [see, L. J. M. J. Blomen and M. N. Mugerwa, "Fuel Cell System", 397, Plenum Press, New York (1993)].
Second, the method of suppressing cathode dissolution by adjusting the composition of carbonates which are used as electrolytes in MCFC was suggested. Since NiO is dissolved in the electrolytes at the common operating conditions of MCFC in accordance with the acidic dissolution mechanism, the method of increasing the basicity of the electrolytes was suggested as a means for lowering the solubility of NiO. Either increasing the content of Li.sub.2 CO.sub.3 or using Na.sub.2 CO.sub.3 instead of K.sub.2 CO.sub.3 was suggested to increase the basicity of 62Li.sub.2 CO.sub.3 -38K.sub.2 CO.sub.3 eutectic salt electrolyte which has been the most widely used electrolyte in the research conducted by several entities including Ota et al. [K. Ota, S. Mitsushima, K. Kato and N. Kamiya, Proceedings of the Second Symposium on Molten Carbonate Fuel Cell Technology, Ed. by J. R. Selman, The Electrochemical Soc., Pennington, N.J., 318-337 (1990)], the ERC (Energy Research Corporation) of the U.S.A. [P. Pigeaud, C-Y, Yuh and H. Maru, 1988 Fuel Cell Seminar Program and Abstracts, Long Beach, Calif., 193-197 (1988)], the GIRIO (Government Industrial Research Institute, Osaka) of Japan [K. Tanimoto, Y. Miyazaki, M. Yanagida, S. Tanabe, K. Kojima, N. Ohtori, H. Okuyama and T. Kodama, J. of Power Sources, 39, 285-297 (1992)] and so on. The research reports of the foregoing organizations show that the experiments changing the composition of the electrolyte from 62Li.sub.2 CO.sub.3 -38K.sub.2 CO.sub.3 to 72Li.sub.2 CO.sub.3 -28K.sub.2 CO.sub.3 or 52Li.sub.2 CO.sub.3 -48NaCO.sub.3 molten salt reduced the solubility of NiO in the electrolyte.
As an another method to increase the basicity of the electrolyte, the addition of a new third basic substance to the electrolyte while maintaining the common composition of the electrolyte was suggested. For example, the fact that the solubility of NiO was reduced compared to conventional cases when the carbonate of an alkaline earth metal (MgCO.sub.3, CaCO.sub.3, SrCO.sub.3, BaCO.sub.3) was added to 62Li.sub.2 CO.sub.3 -38K.sub.2 CO.sub.3 electrolyte was shown by the research results of the ERC of the U.S.A. and the GIRIO of Japan [see the cited literature above], the IGT (Institute of Gas Technology) of the U.S.A. [P. A. Shores, J. R. Selman and E. T. Ong, 1989 Proceedings of the First Annual Fuel Cells Contractors Review Meeting, Ed. by W. J. Huber, Morgantown, W. Va., 161-182 (1989)] and the Toshiba Company of Japan [H. Ohzu, T. Ogawa, Y. Akasaka, M. Yamamoto and K. Murata, 1988 Fuel Cell Seminar Program and Abstracts, Long Beach, Calif., 291-295 (1988)].
Third, the improvement of NiO and the development of substitute substances thereof were suggested. For example, the ANL (Argonne National Laboratory) of the U.S.A. suggests LiFeO.sub.2, LiMnO.sub.2, etc. [L. Smith, G. H. Kucera and A. P. Brown, 1988 Fuel Cell Seminar Program and Abstracts, Long Beach, Calif., 188-192 (1989)], the ECN (Netherlands Energy Research Foundation) of the Netherlands suggests LiCoO.sub.2 and perovskite substance [L. Plomp, J. N. J. Veldhuis, E. F. Silters and S. B. van der Molun, J. of Power Sources, 39, 369-373 (1992)], and Ota et al. suggest nickel ferrite [see the cited literature above].
As described above, various methods for solving the dissolution problem of NiO have been suggested. However there are still many problems associated with the practical utilization of the suggested methods. For example, in the method of adjusting the operating conditions of cells, the performance of the cells is reduced and the lengthening of the life of the cells is limited.
In the method of adjusting the basicity of electrolytes, which reduces slightly the solubility of NiO, there is a limit to how much cell life can be increased by only changing the common composition of the electrolytes to increase the basicity of the electrolytes, and a problem that changing the composition of the electrolytes affects the performance of MCFC. Further, experimental results have shown that the method of adding the carbonate of an alkaline earth metal, for example SrCO.sub.3, as a third substance to common electrolytes does not particularly help to lengthen the life of MCFC [H. R. Kunz and L. J. Begoli, Proceedings of the Second Symposium on Molten Carbonate Fuel Cell Technology, Ed. by J. R. Selman, The Electrochemical Soc., Pennington, N.J., 157-168 (1990)]. This method is not an effective solution to the dissolution problem because the ions in molten salts move in opposite directions in accordance with their respective charges according to the potential difference between the anode and cathode which arises when operating MCFC. Thus, high basicity near the cathode is not maintained due to the distribution phenomena of the ions originating from the mobility difference of each ion. In other words, the cations, i.e., K.sup.+, Li.sup.+, Sr.sup.2+, etc., in molten salts move in the direction of the cathode, but the concentration of Sr.sup.2+ ion in the vicinity of the cathode remains very low due to the different relative mobility of the cations and thus, the addition of SrCO.sub.3 loses its effect.
In the method of developing substitute substances of NiO, cathodes using substitute substances provide the desired property that the solubility thereof for electrolytes is below 1/10 of those of common cathodes. However, various problems still must be solved before practical use is possible, such as the deterioration of cell performance due to the reduction of electrical conductivity, the difficulty in preparing large-scale cell due to the deterioration of the mechanical strength of cathodes, the deterioration of cell performance during operation when pressure is applied and so on.